A1 alloy useful as anode and method of making same

ABSTRACT

AN ALUMINUM ALLOY CONTAINING 0.02 TO 0.2 WT. PERCENT HG, 0.1 TO 20 WT. PERCENT ZN AND 80 TO 200 PARTS PER MILLION CU, USEFUL AS A SACRIFICIAL GALVANIC ANODE IN LOW RESISTANCE ENVIRONMENTS. THE SMALL AMOUNTS OF CU RESULT IN INCREASED ANODE LIFE.

United States Patent Oflice 3,582,319 Patented June 1, 1971 Int. Cl. C22c 21/00 US. Cl. 75-141 2 Claims ABSTRACT OF THE DISCLOSURE An aluminum alloy containing 0.02 to 0.2 wt. percent Hg, 0.1 to 20 wt. percent Zn and 80 to 200 parts per million Cu, useful as a sacrificial galvanic anode in low resistance environments. The small amounts of Cu result in increased anode life.

BACKGROUND OF THE INVENTION The present invention relates to an Al alloy useful as an anode for the cathodic protection of metallic structures, such as pipelines or tanks, against corrosion. In particular, it relates to the addition of Cu to an Al-Hg-Zn alloy to achieve an anode of longer life.

Reding (U.S. 3,321,306) discloses an Al-Hg-Zn alloy highly useful as a sacrificial galvanic anode, particularly in a sea water environment, which has a resistivity on the order of 20 ohm-cm. The alloy is characterized by an electrochemical potential of about 1 volt (as measured against a standard saturated KCl calomel reference electrode) and electrochemical efficiency of about 95%.

Some corrosive environments have a very low resistivity, in the range of 2 to 20 ohm-cm., e.g. oilfield brines. While the above-mentioned alloy can be employed as a sacrificial anode for this type of environment, the potential is higher than necessary to protect the installations. This results in excessive current output and leads to an unnecessary short anode life.

A principal object of the present invention is to provide an alloy for use as an anode with a relatively low potential that will therefore have a longer life in a low resistance environment.

A further object is to provide a method of making such an alloy.

THE INVENTION The present invention comprises an aluminum base alloy containing from about 0.02 to 0.2 weight percent Hg, and from about 0.1 to 20 weight percent Zn, and from about 80 to 200 parts per million Cu. The copper lowers the anode potential. The resultant Al-Hg-Zn-Cu alloy is quite suitable for use as a sacrificial anode in very low resistance, e.g. 2 to 20 ohm-cm, environments.

In the practice of the present invention, conventional alloying and casting or fabricating techniques, as practiced by those skilled in the art, can be employed. The alloying constituents can contain those amounts and types of impurities normally found therein. A1 for use in preparing the Al-Hg-Zn-Cu compositions can be of commercial grade, i.e. 99.7% purity, or higher. Such Al base can contain Cu as a normal impurity. In such case, in accordance with the present invention, Cu is added in elemental form or, e.g., as a prealloy with Zn, to the alloy to bring the total Cu level within the prescribed range.

The alloy when used as an anode is electrically connected to the protected metal by techniques known to those skilled in the art, such as by welding, clamping or bolting the anode to the metal.

The following examples will serve to further illustrate the present invention.

EXAMPLES l-7 A series of Al alloys containing 0.05% Hg, 0.5% Zn and varying amounts of Cu were prepared and cast into cylindrical anodes. These anodes were placed in synthetic sea water in glass jars. A steel wire mesh was placed adjacent to the inner wall of each jar as a cathode. The cells were completed with respect to electrical circuitry, a rectifier being employed to maintain a constant current through a group of cells in series. The cells were run for 30 days at a current density of 300 milliamps per square foot. The electrochemical potential of each anode (as measured against a standard saturated KCl calomel reference electrode) was recorded and is presented in the table.

Total amounts of copper of less than about 80 p.p.m. has little effect on the anode potential (Examples A-L). However, total amounts of Cu above 80 ppm. elfectively control the anode potential (Examples 1-9). This leads to longer life in low resistance environments. Cu in total amounts above 200 ppm. generally offer no additional advantage and may have a substantial adverse etfect on anode efliciency.

In a similar manner Cu can be added within the range disclosed to other Al-Hg-Zn alloys to control the electrochemical potential. The resulting anodes have increased life in low resistance environments.

What is claimed is:

1. An alloy consisting essentially of aluminum containing alloying elements of from about 0.02 to 0.2 weight percent Hg, from about 0.1 to 20 weight percent Zn, and from about 80 to 200 parts per million Cu.

2. A method of making an Al alloy for use as a sacrificial galvanic anode in a low resistivity environment which comprises adding Cu to an aluminum alloy containing from about 0.02 to 0.2 weight percent Hg and from about 0.1 to 20 weight percent Zn, in an amount to produce from about 80 to 200 parts per million Cu in the alloy.

References Cited UNITED STATES PATENTS 3,321,306 5/1967 Reding et al. -146 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75-146 

